The present invention relates to an image forming apparatus including a single machine function of a printer, a copying machine or facsimile machine, or multi-function thereof, an image forming method, and a sheet material conveyance device used therein, particularly to the art of improving the productivity by correcting the curl of the sheet material with an image formed thereon, and contributing to formation of great quantities of images by increasing the conveying speed.
When a sheet material on which an image has been formed passes through the fixing unit of an image forming apparatus, the sheet material will be curled during the passage through the fixing unit. One of the conventional ways of correcting the curl uses a curl correcting member, wherein the sheet material is conveyed by turning the belt with a conveying roller, and to pressing the sheet material conveyed by the belt in one direction using a rotatable pressing roller, whereby the sheet material is interposed and uncurled. If this procedure fails to correct the curl, the direction of conveyance is switched and the surface of the sheet material pressed by the pressing roller is changed. Then the sheet material is again passed through the curl correction device, whereby the sheet is uncurled (Patent Document 1, i.e., Official Gazette of Japanese Patent Tokkaihei 10-114454).
According to another conventional way of correcting the curl, a sheet material is interposed between a conveying member and a curling member during the conveyance wherein the ends of the sheet material are held. In this state, the conveyance speed of sheet material is reduced, and the sheet is uncurled (Patent Document 2, i.e., Official Gazette of Japanese Patent Tokkaihei 11-193160).
Separately from the aforementioned conventional art, it is required to increase the conveying speed and to improve productivity. When these requirements are to be met by the conventional image forming apparatus, it will be necessary to increase the conveying speed using a curl correcting mechanism and to install another conveyance mechanism on the downstream side thereof. When the art of the Patent Document 1, for example, is used to meet the former requirement, the sheet material is interposed (pressed). When this pressure has reached a predetermined level, the conveying speed is increased. This procedure allows a curl to remain on the trailing edge of the sheet material. To meet the latter requirement, the size of the apparatus has to be increased by the addition of another conveyance mechanism. These problems remain unsolved when the aforementioned requirements are to be met.